Process of casting with downward-unidirectional solidification

ABSTRACT

Unidirectionally solidified castings are produced by inverted solidification using a chill plate at the top of the mold and controlling the temperature gradient to cause solidification to occur from the top downwardly through the mold.

United States Patent Inventors Stephen M. Copley Madison; Anthony l Giamei, New Haven, both of, Conn. Appl. No. 872,562 Filed Oct. 30, 1969 Patented Aug. 10, 1971 Assignee United Aircraft Corporation East Hartford, Conn.

PROCESS OF CASTING WI'l'll DOWNWARD- UNlDllI-ZCTIONAL SOLIDIFICATION 4 Claims, 3 Drawing Figs.

Int. Cl. 822d 25/06 Field of Search" 164/60 Primary Examiner- Robert D. Baldwin Attorney-Charles A. Warren ABSTRACT: Unidirectionally solidified castings are produced by inverted solidification using a chill plate at the top of the mold and controlling the temperature gradient to cause solidification to occur from the top downwardly through the mold.

PROCESS OF CASTING WITH DOWNWARD- UNIDIRECTION AL SOLIDIFICATION BACKGROUND OF THE INVENTION Unidirectionally solidified castings of the columnar-grained type as described in VerSnyder U.S. Pat. No. 3,260,505 and also of the type described in Piearcey, Ser. No. 540,1 l4, filed Feb. 17, 1966, U.S. Pat. No. 3,494,709 and assigned to a common assignee, are generally formed by casting in a mold resting on a chill plate with a control of the thermal gradient to cause a controlled upward movement of the liquid-solid interface from the chill plate upward to the top of the mold. Such castings are generally satisfactory although at times there is a density inversion during solidification that creates imperfections within or on the surface of the castings.

SUMMARY OF INVENTION One feature of this invention is the inverted unidirectional solidification of alloys in the mold for the purpose of eliminating the imperfections resulting from the density inversion. Another feature is a mold assembly that makes possible the directional solidification of alloys from top to bottom with a controlled thermal gradient thereby to produce the desired crystalline structure and/or grain growth.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical-sectional view through a mold construction.

FIG. 2 is a horizontal-sectional view along the line 2-2 of FIG. 1.

FIG. 3 is a fragmentary view ofa modification.

DESCRIPTION OF THE PREFERRED EMBODIMENT The shell mold construction includes a central-vertical feeder tube connected at the bottom by lateral arms l2 having passages 14 thereon. Atthe outer ends of the arms are vertical mold elements 16 extending parallel and in spaced relation to the feeder tube and having an article-forming cavity 18 therein. This cavity is shown as being the shape of a turbine blade with an airfoil portion 20, a shroud portion 22 at the bottom end and a root portion 24 at the top thereof. The shroud portion communicates through a vertical passage 30 with the horizontal passage 14. The root portion of the article cavity 18 communicates through a helical-crystal selector passage 32 with a growth zone cavity 34. A chill plate 36 is secured to the mold at the top end of the cavity 34, the mold having a flange 38 at this point by which the chill plate is attached.

As shown in FIG. 2, the mold is generally made up ofa plurality of vertical mold elements 16 so that a plurality of articles may be cast at one time. The mold may be made with a plurality of supporting feet 40. The mold is made by the usual shellmold technique using a wax or other disposable pattern over which successive layers of mold material are placed and dried, with the finished mold cured by heating at which time the pattern is melted out.

When the mold is ready for use, it is placed on a support plate 42 resting on a bed of heat-insulating powered material 44 such as aluminum oxide. The mold is surrounded by a susceptor 46 which in turn is surrounded by vertically spaced induction heating coils 48 and 49 which are selectively energized for controlling the temperature ofthe mold. The susceptor and heaters extend at least from a point above the chill plate to a point substantially below the shroud portion to assure an accurate control ofthe thermal gradient from the chill to the bottom end ofthe vertical passage 30.

The central tube is surrounded by a heating coil 50, preferably a resistance coil, and this coil is shielded from the surrounding article-forming portions of the mold by a cylindri' cal shield 52. A graphite sleeve 53 may be positioned within the coil, as shown. The latter and the shield 52 may rest on the lateral arms 12 and the coil may be incorporated in this shield, as shown. A sprue 54 positioned at the top of the tube directs molten alloy into the vertical passage defined by the tube. The entire structure is preferably mounted in a vacuum chamber so that the casting procedure may be done under vacuum or in an inert atmosphere.

The assemblage is used for example in producing parts such as turbine blades or vanes which operate in a high-temperature environment under high stress and are thus cast from the so-called super alloys such as described in the VerSnyder U.S. Pat. No. 3,260,505 or the Piearcey U.S. Pat. No. 3,494,709. The article-forming portion of the mold and the adjacent portions thereof are heated to a temperature above the melting temperature of the alloy and the central tube is also raised to this same temperature. A flow of water is maintained through the chill plate so that it is not melted during this heating process. When temperatures are stabilized the molten alloy, somewhat superheated, for example, about 50 C., is poured into the sprue to fill the mold completely. A suitable vent 56 in the chill plate permits the escape of entrapped gases within the mold to assure complete filling.

At the time the mold is filled, solidification of the alloy begins at the water-cooled copper chill plate. Up to this time both induction heating coils 48 and 49 have been energized but as the mold is filled the upper coil 49 is cut off to begin the cooling of the mold from top to bottom. The escape of heat to the chill plate causes the vertically downward growth of columnar grains of alloy in the growth cavity 34 and into the helical passage 32 where a single crystal is selected to continue its growth downwardly into the article-forming cavity thereby forming a single crystal article as described and claimed in Piearcey U.S. Pat. No. 3,494,709.

The maintenance of heat from the resistance coil around the central tube keeps the alloy in this tube molten, and the heat from induction coil 48 keeps the alloy molten in the horizontal passages so that the metal in the central tube maintains a positive hydrostatic pressure on the liquid-solid interface where the alloy is solidifying downwardly in the article cavity. This hydrostatic pressure increases as the liquid-solid interface proceeds downwardly and thus the pressure on the mushy zone increases toward the bottom end of This the article cavity.

As the liquid-solid interface moves downwardly during the casting operation, the power in the lower heater 48 is reduced at a programmed rate determined by the rate of solidification, and is then cut off to permit completion of the solidification. Once the downward solidification in the article-forming portion is complete, heat to the central tube heater is cut off so that the remainder ofthe alloy may solidify.

It has been found that in the usual formation of directionally solidified articles from bottom to top there are liquid jetsof the alloy that flow upwardly within the mushy zone caused by instability resulting from a density inversion in this zone. These jets detrimentally affect the proper solidification of the alloy within the mushy zone. This may result in local areas of segregation trails rich in rejected solute which on the surface of the cast article are referred to as freckles. These trails often contain small randomly oriented grains and are a defect which may make the cast article unacceptable. Such freckles or trails would be eliminated by the inverted solidification above described, since the mushy zone density profile will be stable with respect to the gravitational field.

Referring now to FIG. 3, the mold may be adapted for making columnar-grained articles as in the VerSnyder patent. To accomplish this, the growth cavity 34' communicates directly with the root portion 24' so that the columnar growth that is started at the chill plate 36' and becomes parallel vertically oriented columnar grains in the growth zone are propagated downwardly through the article-forming portion. Casting of such articles by this inverted solidification process would be used to produce acceptable columnar-grained articles such as turbine blades for use in the highest temperature turbine stages of the engine.

We claim:

1. In the production of directionally solidified cast alloy articles, the steps of:

providing a mold having vertically extending filling and article-forming cavities interconnected at the bottoms thereof, both cavities being open at the top ends,

placing a chill plate on the open top end of the article-forming cavity,

heating the mold to a temperature above the melting point of the alloy,

filling the mold with the molten alloy,

gradually reducing the temperature of the article-forming cavity in a direction downwardly from the chill plate to cause directional solidification of the alloy within the artiole-forming cavity in a direction downwardly from the chill plate and,

maintaining the alloy in the filling cavity above the melting temperature until the alloy in the filling cavity is solidified.

2. The process of claim 1 with the added step of resting the mold on a bed of heat-insulating material before the alloy is poured into the mold.

3. The process of claim 1 wherein the filling step includes filling the mold such that the level of the alloy in the filling cavity is above the chill plate to provide a positive pressure of alloy against the chill plate.

4. The process of claim 1 with the added step of providing a plurality of vertically spaced heating coils around the article forming cavity and wherein the step of gradually reducing the temperature includes selectively reducing the heating effect of these coils from top to bottom to produce a thermal gradient from top to bottom of the article cavity during solidification of the alloy. 

1. In the production of directionally solidified cast alloy articles, the steps of: providing a mold having vertically extending filling and article-forming cavities interconnected at the bottoms thereof, both cavities being open at the top ends, placing a chill plate on the open top end of the article-forming cavity, heating the mold to a temperature above the melting point of the alloy, filling the mold with the molten alloy, gradually reducing the temperature of the article-forming cavity in a direction downwardly from the chill plate to cause directional solidification of the alloy within the articleforming cavity in a direction downwardly from the chill plate and, maintaining the alloy in the filling cavity above the melting temperature until the alloy in the filling cavity is solidified.
 2. The process of claim 1 with the added step of resting the mold on a bed of heat-insulating material before the alloy is poured into the mold.
 3. The process of claim 1 wherein the filling step includes filling the mold such that the level of the alloy in the filling cavity is above the chill plate to provide a positive pressure of alloy against the chill plate.
 4. The process of claim 1 with the added step of providing a plurality of vertically spaced heating coils around the article forming cavity and wherein the step of gradually reducing the temperature includes selectively reducing the heating effect of these coils from top to bottom to produce a thermal gradient from top to bottom of the article cavity during solidification of the alloy. 